1. Field of the Invention
The invention relates to a process for the generation of process steam and boiler feed water steam in a heatable reforming reactor for the production of syngas. The inventive process serves to exploit the sensible heat of a syngas produced from hydrocarbons and steam so to obtain two types of steam, each being generated when heating and evaporating boiler feed water and process condensate, with the process also including a conversion of the carbon monoxide contained in the syngas, and with the process including optional heating of the boiler feed water by means of the flue gas obtained from the heating of the reforming reactor. The process allows exploiting the sensible heat of the syngas and the flue gas from the heating more efficiently, while avoiding the disadvantages of the flue gas heating on account of the varying amounts of heat available in the flue gas duct. The invention also relates to a contrivance for carrying out this process.
2. Description of the Related Art
Syngas can, for example, be generated by the catalytic conversion of gaseous or evaporated hydrocarbons by means of steam in a heatable reforming reactor, heating being carried out by combusting a fuel gas with an oxygenous gas. Upon generation the syngas has a temperature of approx. 800° C. to approx. 900° C. The sensible heat of the obtained syngas can thus be used for steam generation. Heating yields a flue gas which also carries sensible heat and can equally be used for steam generation. The steam can, in turn, be used for operating auxiliary units or a steam turbine.
For steam generation the process condensate can be used as feed water, which is condensed water forming when the syngas is cooled. This process condensate, however, involves the disadvantage that it contains the same impurities contained in the syngas. Such impurities are frequently of unwanted corrosive effect so that the steam is not unlimitedly suitable for all applications. This steam is thus usually employed as starting steam in the reforming reaction.
In addition, the amount of steam obtained from the process condensate is usually not enough to operate all secondary units which frequently require a constant amount of steam. To solve the problem, additional steam can be generated from clean boiler feed water. Such steam does not contain any impurities so that it meets the strict requirements to be fulfilled by the operation of steam turbines. Hence two types of steam are obtained.
Operation of two steam systems involves great advantages. It is possible to mix the steam from the process condensate with, for example, steam from the boiler water to ensure the availability of a sufficient amount of steam for the syngas production or to influence the steam composition according to the purity required for the process or the downstream application.
The steam obtained from the boiler feed water cannot only be used for the operation of auxiliary units or steam turbines but can also be exported or used as feed steam for the generation of syngas. The steam from the boiler feed water can, for example, be generated by heating the boiler feed water with the process gas, which is freshly produced syngas of high temperature. Analogously, the steam from the process condensate, which is water condensed from the syngas, can be generated by heating the process condensate with syngas. Here, the typical procedure is to heat the boiler feed water or the process condensate in a pre-heater designed as heat exchanger and then to evaporate the heated water in a steam generator. The steam generator can, for example, be designed as a steam drum which is heated by the syngas via fluid-conveying heat exchanger coils.
WO 2010051900 A1 teaches a process and a contrivance for the utilisation of heat in the steam reformation of hydrocarbonaceous feedstocks by means of steam, in which a steam reformer is used to generate a syngas which carries an amount of heat, including at least six heat exchangers, a water treatment unit, a cooling section, a high-temperature conversion unit, at least two pressure-boosting units, at least one consumer and at least one unit for further processing of the syngas obtained, with the generated syngas carrying the first amount of heat passing the high-temperature conversion unit, where its major part is converted to carbon dioxide and hydrogen, and the resulting heat-carrying syngas being routed into a first heat exchanger for further heat transfer, then passing through at least two more heat exchangers which are operated as boiler feed water pre-heaters, product condensate heat exchangers or low-pressure evaporators and are connected in series in any order desired, the syngas resulting from the low-pressure evaporator first being routed to another boiler feed water pre-heater in which heat energy is transferred to a partial stream of the boiler feed water from the water treatment unit, the resulting syngas subsequently passing the cooling section where the syngas is further cooled and a condensate flow produced, and the resulting syngas being finally passed through at least one unit for further processing. The process does not teach any possibility to exploit the heat of the syngas upstream of the high-temperature conversion unit.
To generate the steam from the process condensate, it is also possible to use the sensible heat of the flue gas. US 2009242841 A1 teaches a process for the generation of syngas in which the syngas is generated by steam reforming in a reforming reactor, with a combustion air flow, a convection zone and a flue gas stream and the process including the process step of combustion air passage through a preliminary heat exchanger system in the convection zone in order to heat the combustion air in indirect heat exchange with the flue gas, with the temperature of the pre-heated combustion air ranging between approx. 93° C. (200° F.) and 204° C. (400° F.). According to an embodiment of the process, boiler feed water is heated by passing it through the syngas cooling section and the combustion air convection zone after or in parallel to the combustion air to be heated, the convection zone being heated by the flue gas stream.
To heat the boiler feed water or the process condensate by the flue gas, it is usually necessary for part-load operation to adapt the heat amount available in the flue gas duct at constant mass flow of process condensate or boiler feed water in order to ensure evaporation of the water. This means that an additional amount of heat is to be provided at least temporarily by means of auxiliary burners, for example. This involves increased operating cost.
As the dual steam system, however, involves the aforementioned advantages, possibilities are explored to achieve further improvements. A starting point for improving the efficiency of the dual steam system is to bypass the heat exchanger in the flue gas duct for the period of time during which there is not enough heat available in the flue gas duct. In this way, there is no need to operate additional burners for heating the flue gas duct.
It is possible to heat the process condensate by the waste heat of the flue gas duct. However, as the process condensate is usually not cooled down to the temperatures of the cool boiler feed water, its temperature is higher than that of the cool boiler feed water. Owing to the lower temperature difference to the flue gas, the heat exchangers for the process condensate in the flue gas duct thus require larger heat exchanging surfaces according to Newton's law of cooling. As the temperature difference between the process condensate and the hot syngas is higher, significantly smaller heat exchanging surfaces are required in the process gas line downstream the reforming reactor than in the flue gas duct. As the water-gas shift reaction additionally takes place at considerably lower temperatures than the syngas production, the heat of the syngas production can be used more efficiently for the process condensate right after the discharge from the reforming reactor where the temperature is significantly higher than downstream of the conversion unit. Smaller-sized heat exchanging surfaces, in turn, will contribute to an improved cost-effectiveness of the process.